A locomotive is a complex system with numerous subsystems, each subsystem interdependent on other subsystems. An operator aboard a locomotive applies tractive and braking effort (including the application of dynamic brakes and independent brakes at the locomotive and air brakes at the railcars of the train) to control the speed of the locomotive and its load of railcars to assure proper operation and timely arrival at the desired destination. Speed/power control must also be exercised to maintain in-train forces within acceptable limits, thereby avoiding excessive coupler forces and the possibility of a train break. To perform this function and comply with prescribed operating speeds that may vary with the train's location on the track, the operator generally must have extensive experience operating the locomotive over the specified terrain with different railcar consists.
Train control can also be performed by an automatic train control system that determines train and trip parameters, e.g., train mass, train location, and applies tractive effort and braking effort to safely and efficiently control the train. Alternatively, a train control system can advise the operator of preferred train control actions, with the operator manually exercising train control in accordance with the advised actions and in further accordance with his/her independent train control assessments.